In recent years, plastic electronics has grown in importance in an increasing number of areas, particularly in display applications used in a wide range of consumer handheld electronic devices containing miniaturised display screens. The goal of achieving an all-polymer electronics device has motivated research groups around the world to provide plastics—which are known for their low-cost processing, flexibility and toughness—with the electronic properties needed for practical circuitry in a wide range of electronic devices.
Organic luminescent electronic devices (OLEDs) are one example of plastic electronic devices which have grown to become a promising technology for flat panel displays, relying on organic thin-films to produce light. The active layer in an OLED is a fluorescent organic material that emits light when an electrical current passes through it. The fluorescent organic material is also known as an electro-luminescent (EL) element and is arranged between a transparent anode layer and a transparent cathode layer. It is known that the fluorescent organic material comprises organic molecules that are reactive towards moisture and oxygen, thus causing the EL element to degrade over time due to reaction with moisture and oxygen.
In order to protect the organic EL element from degradation, the organic EL element is typically enclosed within an encapsulation that seals off moisture and oxygen. Attempts have been made to provide various types of encapsulation structures that are resilient towards handling and effective in providing moisture and oxygen barrier. Conventional types of encapsulation, such as that described in European Patent Application No. 0 776 147 A1, involve placing the moisture and oxygen sensitive components within an enclosure defined between two substrates that have low gas permeation properties. The two substrates are held together by adhesives, with desiccants placed within the enclosure to absorb moisture and oxygen. In some instances, an inert gas such as nitrogen may be sealed within the enclosure.
Current efforts in encapsulation technology are focused on lowering the gas permeability of the base substrate on which the OLED is formed. For example, US Patent Application No. 20030203210 (Vitex Systems, Inc.) describes substrates that comprise alternating polymer and metal oxide inorganic layers placed on a supporting substrate. The metal oxide inorganic layers were found to demonstrate low permeability towards gases, and when used in an alternating layer arrangement, low gas permeation rates were achieved. However, inorganic metal oxide layers have an inherent structural problem in that surface defects such as pinholes and cracks are formed in the layer during fabrication. These surface defects provide a pathway for water and oxygen entry, thereby compromising the gas permeation properties of the encapsulation. Furthermore, metal oxide layers are prone to developing further structural defects when mechanical flexing stresses are applied.
U.S. Pat. No. 6,465,953 (General Electric Company) describes a transparent polymer substrate that is filled with moisture getter particles. The substrate is formed by adding moisture getter particles, such as BaO and MgO, to thermosetting polymers such as polycarbonate. The plastic substrate is coated with an additional inorganic film comprising SiO2 or Si3N4 to improve the barrier properties of the substrate.
Other than improvements to the base substrate, alternative approaches to improving the gas barrier properties of the encapsulation are seldom explored. In order to achieve device lifetimes of above 30,000 hours, current efforts in encapsulation technology are still focused on designing barrier substrates that can achieve low water vapour permeation rates of 10−5 g/cm2/day or less.
An object of the present invention is to provide a flexible multilayer barrier film capable of encapsulating a moisture and oxygen sensitive electronic device which can be used in conjunction with any existing encapsulation design.